1. Field of the Invention
This invention relates to improvements over the system disclosed in U.S. Pat. No. 5,179,610, in which radiation that fails to couple,with a small core fiber is transmitted via a secondary transmission path to a “dissipation chamber,” a reflector, and an external heat sink that converts the reflected radiation to heat.
The first improvement is to use diffusion to spread-out the errant radiation, thereby minimizing damage caused by the radiation and eliminating the need for a reflector or external heat sink. As a result, the size and cost of the termination is reduced, and the termination can be fitted into existing fiber optic termination apparatus without the need for substantial modification. While an internal heat sink may be included, the heat sink can be made relatively small since it is only required to absorb diffuse radiation.
The second improvement is to strip away or reduce the thickness of the cladding at the coupling end of the fiber to reduce or eliminate launching energy into the cladding of the fiber.
The third improvement is to taper a section of the fiber core and/or cladding to decrease higher order radiant energy propagation modes within the conducting medium.
2. Description of Related Art
a. Apparatus to which the Invention may be Applied
The method and apparatus of the invention is especially suitable for use in connection with (but not limited to) relatively small primary optical transmission systems that employ optical fibers, and in particular medical devices such as scalpels or lithotripter fibers, which are typically, but not necessarily, monochromatic. Such optical fibers are especially useful to implement recently developed, minimally invasive surgical techniques.
An example of an apparatus to which the principles of the invention may be applied is illustrated in FIGS. 1 and 2. The apparatus includes a laser system 12 and a standard connector coupler 18 for coupling a connector such as connector 30 shown in FIG. 2, which couples the output of the laser system to a primary optical system such as an optical fiber or fiber cable 32. The laser may be a high energy pulse or continuous wave laser that generates a monochromatic radiant energy output beam 17. For example, the laser system may be a Holmium:YAG laser that generates an output formed of pulses on the order of 250μ seconds in pulse width and energy levels ranging up to 1800 mj/pulse with an average power of 12 Watts. The output beam 17 is passed through a condensing lens to form an output beam 17a that is focused on a spot 17b in the vicinity of input focal plane 16 and centered in a connector coupler 18 mounted on the laser enclosure 15 using respective X, Y, and Z adjustments 12a–12c. When connector 30 shown in FIG. 2 is secured to connector coupler 18 by locking member 40 (which may, for example, be an internally threaded nut), the connector ferrule 31 is ideally centered on focused spot 17b and the distal end of the connector ferrule is at the focal plane 16. In this example, the focused spot size at the focal plane 16 is on the order of 365 microns and the relative power density at the focal plane for a 365 micron spot with an average power of 12 watts is approximately 11.5 kW/cm2. On the other hand, the power density 6 mm beyond the focal plane 16 is reduced by a factor of 50.
Ferrule 31 of connector 30 is typically a metal elongated hollow body member into which is inserted the optical fiber or fiber cable 32. The proximal end has a fiber clearance hole 38 drilled close to the outside diameter of the fiber 35. To secure the fiber 35 to the ferrule 31, a small portion of the fiber optic cable 32 is stripper away exposing the glass fiber 35. Before the stripped fiber is placed inside ferrule 31, an adhesive 34 may be applied to a small portion of exposed fiber 35 and the exposed fiber is passed through the internal diameter of the ferrule to its distal end. The extreme distal portion of the exposed fiber exits the ferrule through fiber clearance hole 38. Later, after adhesive 34 is cured, the exposed fiber 35 is trimmed and polished such that the distal end of the ferrule and the distal end of the fiber 35 are flush. Alternately, the fiber 35 may be secured within the ferrule by crimping a portion of the cable 32 to the ferrule using a sleeve.
b. The Errant Radiation Problem
The errant radiant energy problem arises when the primary optical transmission system approaches or is smaller than the size of the focused beam of radiant energy. For example, the smaller the diameter of an optical fiber, the more difficult it is to focus energy from the laser into the core. If the core diameter is smaller than that of the focused spot of the laser source, or if the focused radiant energy to the core is misaligned or greater than the fiber's acceptance angle, then energy will be transferred to structures that make up the coupler or that surround the core. The density is often great enough to soften, melt, or fuse any materials which are not highly optically transmissive or reflective. In many cases the energy density can be so great that photo thermal ablation may occur in the metal housing of the connector that couples the laser to the fiber, causing the metal to explosively form a plume mixture gases and micron size particles, which re-deposit and contaminate the focusing lens. Further lasing into the contamination can create extreme localized heating which ultimately destroys the focusing lens.
One solution to the problem of errant radiant energy is to divert the radiant energy along a second transmission path to a heat sink situated in, or comprising, the connector support structure, as disclosed in U.S. Pat. No. 5,179,610. As a result, it is necessary to modify the support structure to serve as a heat sink, and to provide a reflector capable of withstanding the radiation transmitted thereto by the secondary transmission path. The secondary transmission path is required to be highly transmissive (see col. 4, lines 31–39 of U.S. Pat. No. 5,179,610) and may be in the form of a quartz, glass, or Zirconium Fluoride collar having some heat dissipating effects, but primarily provided to guide the errant radiation to the reflector, while the reflector may be in the form of a multifaceted polished metal structure interference fitted into the connector, and the heat sink may be in the form of a metal element bolted to the fiber optic connector at an appropriate position.
The arrangement disclosed in U.S. Pat. No. 5,179,610 can, in theory, effectively divert radiation away from components of the fiber optic termination or connector, but implementation of the arrangement requires replacement of the conventional supporting structure, making it difficult to adapt the arrangement to existing fiber terminations. Further, the arrangement requires a reflector capable of withstanding the radiation to be dissipated, and a supporting structure that can effectively convert the radiation to heat and dissipate it without exposing the user or other surrounding structures to the resulting heat. As a result, the arrangement disclosed in U.S. Pat. No. 5,179,610 does not appear to be compatible with existing laser systems and connectors, and in particular does not appear to be well-adapted for use with standard medical laser industry connectors, such as the SMA 905 standard connector.
There is therefore a need for a coupling apparatus and method that minimizes the impact of radiant energy that fails to couple to the core of the optical fiber (or other primary optical transmission system), and further that can be fitted into existing termination arrangements, without the need to add a heat sink or modify the termination to safely dissipate the radiation. While heat sink structures may be utilized in the termination arrangement of the invention, they are designed to fit within the termination, and do not require a reflector.
c. The Problem of Coupling to the Cladding
In addition to dissipating radiant energy that fails to couple with the fiber, the present invention addresses the problem that, even when all of the energy that fails to couple to the fiber is dissipated, some of the energy that couples to the fiber will couple to the cladding of the fiber rather than to the core, causing the cladding to act as a secondary wave guide and leak energy into surrounding coating during tight bends, such as my occur when the optical fiber is used for laser lithotripsy after it has been passed through the working channel of an endoscope. While the amount of coupling may be reduced by tapering, the core and cladding may mix, causing light to also mix into the cladding, and higher order modes may be created which are more subject to loss during a bend than lower order modes. By way of background, it was proposed in U.S. Pat. No. 6,282,349 to fuse the cladding to the ferrule in which it is placed, but the cladding fusion scheme described in this patent did not involve removal of some or all of the cladding at the end of the fiber reduce coupling of laser energy into the cladding.
An improvement over the arrangement of U.S. Pat. No. 5,179,610 is disclosed in copending U.S. patent application Ser. No. 10/370,453, filed Feb. 24, 2003, by the same inventor as the present application. This copending patent application describe a laser-to-fiber coupling arrangement that can be used without an external heat sink, by reflecting the errant laser energy back into the laser itself. This has the advantage of not only eliminating the heat sink, but also of enabling use of a less expensive and more reliable single facet reflector. In addition, the patent application discloses the concept of roughening the end surface of a transparent ferrule to diffuse incoming radiation so as to reduce the density of radiation incident on the reflector. However, while this represents an improvement over the arrangement disclosed in U.S. Pat. No. 5,179,610, elimination of the reflector entirely would have the additional advantages of shortening the secondary transmission path, simplifying manufacture, and increasing the useful life of the apparatus. There is still a need for a laser-to-fiber coupling arrangement that reduces or eliminates coupling of focused radiant energy into the cladding of an optical fiber, rather than into the core, and yet does not require an external heatsink or any sort of reflector.
b. The Problem of Higher Order Modes
When light propagating down a fiber core exceeds the critical angle created by the core/cladding interface due to tight bends in the fiber, light will leak from the core into the adjacent cladding and outer coating surfaces. Damage to the cladding or coating will occur if these surfaces are unable to withstand the incident power or energy density. Continued damage to these surfaces can ultimately lead to total destruction or breakage of the overall fiber. Since higher order radiation modes are by definition modes in which the angle of incidence is relatively high, higher order modes are more likely to exceed the critical angle at a bend in the fiber, and therefore leak to the cladding.
There are two conventional ways to deal with this problem. The first is to use the largest fiber numerical aperture (N.A.), thereby increase the acceptable angles at which energy from the source propagates within the light conducting medium. The second is to reduce the input numerical aperture from the source, which also has the effect of increasing the acceptable propagation angles. However, both solutions are undesirable for many applications. Larger N.A.s are limited, due to choices of fiber core/clad materials versus transmission at a particular wavelength. The N.A. of fiber is determined by the refractive indices of the core/clad materials.N.A.2=(ncore2−nclad2)Therefore, choices of larger N.A. fibers are limited in many state of the art minimally-invasive surgical techniques involving, for example, scalpels or lithotripter fibers, while decreasing the numerical aperture of the radiation source is often impractical due to the already large installed base of lasers, theoretical limitations and FDA regulations on making modifications to existing lasers. The present invention is especially useful in such cases, i.e., where it is necessary for the fiber to effectively reduce the source N.A., although the invention can be used in any context where propagation of higher order radiant energy modes is of concern.